1. Field of the Invention
This invention relates to a method for making cured poly(glycidyl nitrate) having high stability against de-cure. This invention also relates to the production of solid energetic compositions, such as propellants, explosives, pyrotechnics, and gas generants, comprising poly(glycidyl nitrate) binders.
2. Description of the Related Art
Solid high energy compositions, such as propellant, explosives, pyrotechnics, and gasifiers, or the like, generally comprise solid particulates, such as fuel particles and/or oxidizer particles, dispersed and immobilized in a cured binder matrix.
In recent years, energetic polymers have been developed and evaluated as replacements for inert polymeric binders in cast propellant systems, explosives compositions, and pyrotechnics. The substitution of an energetic polymer for an inert polymer in a typical pressable or extrudable explosive composition generally increases the detonation pressure and detonation velocity of the explosive.
Poly(glycidyl nitrate) (also known as “PGN” and “polyGLYN”) has been known and recognized for years as a possible energetic polymer suitable for use in propellants, explosives, gas generants, pyrotechnics, and the like. PGN is commonly synthesized in the industry by preparing a difunctional glycidyl nitrate polymer, and curing the PGN with a polyfunctional isocyanate having a functionality of greater than about 2.3 to give urethane cross-linked polymers. Aliphatic polyisocyanates have been selected as the curing agents.
Although glycidyl nitrate prepolymers have a satisfactory shelf life, it is known that conventionally cured PGN inherently de-cures when subjected to elevated temperatures for prolonged periods. If precautions are not taken, over time cured PGN can de-cure to the point of reverting to a pourable liquid. Accordingly, special care must be taken in the handling and storing of energetic compositions containing crosslinked PGN. The special care required to avoid de-curing problem has impeded the widespread use of PGN as a binder, despite its attractive energetic properties.
One solution to this de-curing problem has been suggested by N. C. Paul et al., An Improved polyGLYN Binder Through End Group Modification, ICI Explosives (1998). The article indicates the de-curing problem as being caused by the proximity of the terminal hydroxyl groups of the polymer to nitrate ester groups. The authors conclude that the de-curing problem is an inevitable consequence of the end group structure. To overcome this problem, the article describes a two-step process that modifies the end groups by removing the adjacent nitrate esters and replacing the nitrate ester groups with hydroxyl groups by base-catalyzed hydrolysis. Aging tests have shown that this technique is successful in preventing de-cure of the polymer. However, one of the major drawbacks to this solution of PGN de-curing is the decrease of energetic performance. It is estimated that approximately 10 percent of the nitrate esters may be removed from the polymer chain in accordance with this technique. Another drawback is that the extra chemical process steps cause additional expense and chemical waste in production.